The invention relates to a method for determining the torque magnitude transferred to a threaded fastener at each one of a number of repeated torque impulses delivered to the fastener by an impulse tool, as well as a device for tightening threaded fasteners by repeated torque impulses, including means for determining the torque transferred to the fastener by determining the retardation magnitude of the rotating parts of the impulse tool.
The invention intends to solve the problem of providing a reliable yet simple technique for determining the torque magnitude transferred to a threaded fastener at each torque impulse delivered by an impulse tool without using a torque transducer and/or an angle sensing means on the output shaft of the impulse tool.
In for instance U.S. Pat. No. 6,134,973 there is described an impulse tool having an output shaft provided with both a torque transducer and an angle encoder. These torque and angle sensing means deliver signals to a control unit where the torque magnitude is determined at the very end of the rotational movement of each impulse, which means that the angle sensor is used for rotational movement indication only. The installed torque is measured by the torque transducer the very instant the fastener stops rotating.
A drawback inherent in this known technique is that the torque transducer arrangement is rather complicated as the output shaft is made of a magneto-strictive material and comprises a portion with a particular surface pattern surrounded by electric coils mounted in the tool housing. Moreover, this torque sensing device together with the angle sensing device add to the length of the output shaft and, hence, the entire tool. A further drawback of this known device is the difficulty to obtain a distortion-free signal from the angle sensor, because the non-rigid socket connection between the shaft and the fastener always tends to cause uneven movements of the output shaft. The step-wise movements of the output shaft during impulse tightening are very short, which means that it is difficult to obtain accurate angle responsive signals.
In U.S. Pat. No. 5,567,886 there is described an impulse tool having a hydraulic pressure activated torque detecting device for tool shut-off purposes and an angle sensing device mounted at the rear end of the motor rotor. The fastener tightening technique described in this prior art document is based on a torque controlled tightening process combined with a result checking step based on the “green window” technique. This means that the torque and angle signals obtained at the end of the tightening process are checked against predetermined limit values for obtaining an o.k.-signal or a not-o.k.-signal.
The technique described in this document is disadvantageous in that it is based on a piston-rod assembly extending out of the hydraulic impulse unit to activate a sensor beam at the rear end of the motor in response to the pressure peaks generated in the impulse unit. A problem concerned with this type of torque sensing device is that seals around movable elements extending out of the hydraulic impulse unit are difficult to get fully leak proof.